The present invention relates to a miniature D.C. (direct current) motor provided with a rotation detector. The detector detects a rotating condition such as revolving speed of the motor utilized for an actuator for driving, for example, a power window of an automobile.
Miniature D.C. motors have been widely applied to automobiles in recent years, and this application tends to increase as the electronics technology progresses. For example, a miniature D.C. motor is utilized for operating the power window which automatically opens and closes window glasses of the automobile. Further the opening and closing operations of the window glasses are performed in a conventional technology by simply opening and closing the window glasses at full length. Recently it has been required to open and close the window glasses in various modes. For example, it requires for window glasses to ascend or descend with small pitches. In other cases the window glass would stop instantly in its ascending operation or open in a safety view point when a hand or a neck of a person is nipped between the window glass and the window frame.
As described above, for the motor utilized for the actuator to drive the power window in various modes, it is necessary to detect the rotating condition of the motor by a rotation detector. The rotating condition such as sum of revolutions of the motor and revolving speed thereof is detected for detecting the positions of the window glasses and the opening and closing speeds.
In the known art, there is a miniature D.C. motor provided with a rotation detector which is accommodated inside the motor. The motor is provided with a casing in which a permanent magnet is mounted and a rotor is disposed in the permanent magnet. The rotor has a rotation shaft supported to be rotatable by two bearings mounted in the casing and a commutator is mounted to the rotation shaft. A brush fixed to the casing is slidingly engaged with the commutator.
A rotation detecting ring of the rotation detector is mounted to the rotation shaft. The rotation detecting ring is disposed adjacent to the commutator and has substantially the same structure as that of the commutator. On the other hand, a rotation detecting brush is attached to the casing and slidingly engaged with the rotation detecting ring. Connecting terminals are provided for a cover of the casing and connected to the brush and the rotation detecting brush respectively through lead wires embedded in the casing. According to these structures, the revolving speed of the rotor is detected through ON-OFF signals transmitted from the rotation detecting ring and the rotation detecting brush constituting the rotation detector.
According to a structure of the conventional miniature D.C. motor, the rotation detecting brush and the ring are disposed inside the casing. Therefore, the inside arrangement or structure thereof is complicated and the axial length of the motor itself is made large in comparison with a miniature D.C. motor having no rotation detector. In nature, it is required to assemble the actuator in which such motor is mounted in a small space. It is not desired to increase the outer dimension of the motor caused by the additional arrangement of the rotation detector. If the motor provided with the rotation detector were designed to be forcibly assembled without changing its outer dimension, this design would adversely affect the arrangement of the commutator and the brush which are essential for the structure of the motor, and would result in degradation of the motor performance.
Further, in the conventional technique the rotation detector is arranged inside the motor casing so that the structures of the motor itself and the numbers of the lead wires are different between the cases of the location of the rotation detector and of no location thereof. It is therefore difficult to commonly use motor parts such as the casing, the shaft and the lead wires in both cases. Moreover, in the case of the location of the rotation detector, it is necessary to embed the lead wires in the casing, accordingly such portions at which lead wires are embedded must be perfectly sealed. Thus the case involves a problem of a water-proof structure which is particularly important for the miniature D.C. motor mounted to the automobile. In the conventional structure, the contact-type rotation detecting ring contacts the rotation detecting brush. It is necessary to pay attention to this portion in view of friction or the like, which may render short the life of usage of these members.